From 1972 to 1992 Delaware used a system of “challenge” primaries, in which a candidate for statewide office who received at least 35 percent of the convention vote could challenge the endorsed candidate in a primary. Democratic nominee Tribbitt avoided such a primary in 1972.[1][2]
Glashan, Roy R. (1979). American Governors and Gubernatorial Elections, 1775-1978. Meckler Books. ISBN0-930466-17-9.{{cite book}}: CS1 maint: ref duplicates default (link)
Gubernatorial Elections, 1787-1997. Washington, D.C.: Congressional Quarterly Inc. 1998. ISBN1-56802-396-0.